Boron-doped diamond film microelectrode will be fabricated, characterized and tested as a novel sensor for neurotransmitters. The remarkable properties of the newly developed diamond electrodes are ideal for biochemical sensing applications, where long term stability, species selectivity, and high sensitivity are required. Diamond is just beginning to be explored as a material for biomedical applications. Polycrystalline and single-crystal diamond coatings will be vapor deposited onto substrates including carbon microfibers, and extensive materials characterization made. Surface modification of diamond electrodes will be performed. The electrochemical response as a function of the diamond film quality, crystallinity, sp2 content, and surface termination will be tested. Initially, dopamine oxidation kinetics, temporal response, and sensitivity will be measured. The surface termination and functionalization of polycrystalline and single- crystal diamond electrodes will be engineered to address issues of sensitivity and selectivity of, e.g., dopamine in the presence of ascorbic acid. Single-cell, in vitro, and in vivo experiments will be conducted and the diamond electrode stability measured. Spatial variations and drug effects will be monitored. Diamond's lower baseline current should make measurement of basal neurotransmission possible. For the first time, long-term in vivo experiments will be possible. Surface modification will allow studies of enzyme-mediated reactions. With diamond's attractive electrochemical properties and well-established fabrication techniques, this project could lead to new applications of diamond film microelectrodes for long-term, in vivo biosensors.